use the correct toolchain "go" tool under GOTOOLCHAIN=auto

We call `go list` to collect information about all the packages
to obfuscate and build, which is crucial to be able to perform
obfuscation of names used across packages.

However, when GOTOOLCHAIN causes a toolchain upgrade,
we must ensure that we use the upgraded Go tool;
otherwise we are mixing information from different toolchain versions.

Fixes #934.

main.go

@@ -2326,6 +2326,7 @@ To install Go, see: https://go.dev/doc/install
 	if err := json.Unmarshal(out, &sharedCache.GoEnv); err != nil {
 		return fmt.Errorf(`cannot unmarshal from "go env -json": %w`, err)
 	}
+	sharedCache.GoCmd = filepath.Join(sharedCache.GoEnv.GOROOT, "bin", "go")
 	sharedCache.GOGARBLE = cmp.Or(os.Getenv("GOGARBLE"), "*") // we default to obfuscating everything
 	return nil
 }

shared.go

@@ -49,6 +49,10 @@ type sharedCacheType struct {
 
 	GOGARBLE string
 
+	// GoCmd is [GoEnv.GOROOT]/bin/go, so that we run exactly the same version
+	// of the Go tool that the original "go build" invocation did.
+	GoCmd string
+
 	// GoVersion is a version of the Go toolchain currently being used,
 	// as reported by "go env GOVERSION" and compatible with go/version.
 	// Note that the version of Go that built the garble binary might be newer.
@@ -292,7 +296,7 @@ func appendListedPackages(packages []string, mainBuild bool) error {
 	}
 
 	args = append(args, packages...)
-	cmd := exec.Command("go", args...)
+	cmd := exec.Command(sharedCache.GoCmd, args...)
 
 	defer func() {
 		log.Printf("original build info obtained in %s via: go %s", debugSince(startTime), strings.Join(args, " "))

testdata/script/gotoolchain.txtar

@@ -4,9 +4,6 @@
 setup-go go1.23.7
 env GOPROXY=proxy.golang.org
 env GOMODCACHE=${HOST_GOMODCACHE}
-# The bug below is about a badly patched cmd/link, so we need a separate
-# GARBLE_CACHE to not allow reusing the host's working patched linker.
-env GARBLE_CACHE=${WORK}/.garble-cache
 
 # Verify that we are using an older version of Go.
 exec go version
@@ -19,9 +16,11 @@ cd mod
 exec go run .
 stderr 'hello from go1\.24\.1'
 
-# TODO: fix this.
-! exec garble run .
-stderr 'gcMarkTermination: relocation target .* not defined'
+# Note that garble hides runtime.Version by design, but we know that it requires
+# the Go toolchain version to match the exact same version that built garble,
+# and that version is most likely newer than GOVERSION_BASE.
+exec garble run .
+stderr 'hello from unknown'
 
 -- mod/go.mod --
 module test